Colour reproduction systems for correcting colour separation printer images



May 23, 1961 D. H. MAWBY 2,

COLOUR REPRODUCTION SYSTEM FOR CORRECTING COLOUR SEPARATION PRINTERIMAGES Filed July 15, 1959 l8 Fig. 5. 5 8 U I I8 Fig. 4 R 1/ l8 F4 5 R u6 I Inventor 29M flbwfi. flaw:

Attorney nitecl States COLOUR REPRODUCTION SYSTEMS FOR COR- figglgg;COLOUR SEPARATION PRINTER David Harry Mawby, London, England, assignorto J. F.

Crosfield Limited, London, England, a British com- P y This inventionrelates to colour reproduction systems of the kind in which a blackprinter is used in addition to the colour printer plates. When a blackprinter is used it is necessary to remove some of the colour from thecolour printer plates, the amount removed being dependent on thatprovided by the black printer. The present invention has for its objectto provide a system for undercolour removal which is simple butflexible.

In electronic colour reproduction systems, the electronic computer haspreviously been designed to compute as accurately as possible theamounts of the cyan, magenta and yellow inks which are required tosimulate in the reproduction the red, green and blue components of theoriginal. Usually, the design has been such that the computer solves theNeugebauer equations for colour reproduction. For a four-colourreproduction system (i.e. one including a black printer) these equationsexpress each additive primary colour component of the original as thesum of sixteen colours, namely white, the three ink colours, the threecolours resulting from an overlap of two coloured inks, the colourresulting from an overlap of the three coloured inks and each of theprevious eight colours together with black. The coefficient of eachcolour except the triple-overlap colour includes expressions defined bythe difference of two terms representing amounts of coloured inks. Thecomputers have therefore included means for adding the correctionsignals in opposite polarity or phase. However, the results obtainedfrom such computers have not been consistently good, and the applicantbelieves that this results from the presence of the difference terms inthe correction signal produced in the computer. In theory, usingdifference terms in the solution of the Neugebauer equations shouldenable all the unwanted density to be removed from the colour separationprinter image. In practice, a serious loss in the quality of thereproduction occurs because contamination is introduced due to smalldefects in the optical system, and noise which is always present invalves and, in particular, in photomultipliers. For instance, particlesof dust which would normally not be noticed may cause small signals inone channel and may not be present in the other, or due to the opticalarrangement the same dust particles may, in fact, be present in theother channel but occur at a different time. In order to achieve a highdegree of correction, the signal in one channel is divided into asubtracted from the signal in the other, and the resultant quotient ordifference signal which is often very small in comparison with thechannel signals (and in which variations due to dust are of appreciablemagnitude), has to be considerably amplified to produce a large changein illumination. Where these small difierences have to be expanded, thepresence of dust particles is very noticeable in the printed result dueto the action of the correction circuits. Although dust particles havebeen used in the above example, grain or other blemishes on any plate ordiffusing surface on or near the object or image planes or on anyreflecting sur- 2,985,712 Patented May 23, 1961 face produce the sameresult. In addition, when a light beam is split through two or morelenses the inherent aberration of the lenses can cause density driftswhich are increased in a similar way. Added to this there is alwayspresent in any circuit some noise, especially where photomultipliers areinvolved, and this too can be amplified to add to the interference ofthe result.

The applicant has found that these defects are substantially avoided ifall the colour correction and undercolour removal signals modulating theexposure of the colour separation printer image are of the samepolarity, the polarity being such that each tends to reduce the densityof a positive image resulting from the exposure. Moreover, the applicanthas found that although in theory the smallest of the colour channelsignals should be selected as being representative of the neutralcomponent present in the scanned element, and should be used forundercolour removal, in practice such as system introduces furtherdifiiculties. When the signals are small, a slight drift in the relativeoperating potentials of the three channels can result in the Wrongsignal being selected. Additional circuits to stabilise voltage levelscan be cumbersome and detract from the reliability of the device as awhole.

According in the present invention therefore the colour separationprinter images are obtained by scanning at least one image constitutedby or derived from the original to obtain a plurality of electricsignals varying with the transmission values of the scanned elements, atleast one of the signals including a component representing undercolorto be removed from the colour separation printer images, deriving fromthe said electric signals correction signals representing the colourcorrection to be applied to the colour separation printer images,modulating a light source by means of an electric signal which variescontinuously with each of the undercolour removal signals from thescanner and which also includes the colour correction signal, andexposing each of the photographic surfaces from which the colourseparation printer images are to be produced to light which originatesat the light source and which at each point on the photographic surfacerepresents the density of the colour to be corrected modified inaccordance with the colour correction and undercolour removal signals,each of which acts in a sense such as to reduce the density of apositive image resulting from the exposure.

This method has been found in practice to produce acceptable andreliable results. The signal designed to efiect the removal ofundercolour is derived throughout the exposure from the same source anddefects arising from relative changes due to potential drifts do notoccur. Moreover, the interference resulting from the presence ofdifference terms does not exist. As a result a small misadjustment ofthe correction circuit does not produce a completely unacceptableresult, as in other systems where the adjustment is necessarily morecritical.

When a single image is scanned to provide the undercolour removalsignal, it may be obtained by exposure to white light transmittedthrough an original colour transparency, or it may be obtained as acolour separation plate from the original, the colour separation platebeing produced by a single exposure to light representing the presenceof a single colour in the original, or by a number of exposures in whichthe light rays represent respectively the presence of a number ofdifferent colours in the original.

According to a subsidiary feature of the invention, a black printer isobtained by scanning an image or images to obtain a colour-componentsignal or signals, and exposing a photographic surface from which theblack printer is to be produced to light which at each point on.

the surface represents the density of a multi-coloured image of theoriginal modified in accordance with the values of the colour-componentsignal or signals so as to reduce the density of a positive imageresulting from the exposure.

In order that the invention may be better understood, an example thereofwill now be described with reference to the accompanying drawings inwhich:

Figure 1 shows diagrammatically a form of imagereproducing apparatuswhich may be used to carry out the method according to the presentinvention; and

Figures 2 to 5 are diagrams illustrating the manner in which theapparatus of Figure l is used.

In carrying into efiect this example of the method according to theinvention an undercolour removal negative is first made by exposure of aphotographic layer to substantially white light transmitted through orreflected from a coloured original, or by exposure to the colouredoriginal through a number of colour filters (for example, through red,green and blue filters in succession). In practice, it may in some casesbe desirable for the exposing light source to have some colour bias. Theundercolour removal negative is then used, for the purpose of removingundercolour from the colour separation plates, in the apparatus ofFigure 1. This apparatus includes a cathode ray tube 6 on the face ofwhich is formed a scanning raster 8 which is imaged on to threetransparencies 10-. Light which passes through the three transparenciesis collected by the three multipliers 12, the output signals from whichare fed to a computer 14. The computer 14 provides four signalsrepresenting the corrections to be applied in producing the photographicplates from which the three colour printers and the black printer aremade. The switch 16 enables one of these output signals to be applied tothe control electrode of the cathode ray tube 6, with the result that acorrection light mask is formed on the face of the tube. Thephotographic plate 18 to be exposed is placed immediately be hind andsubstantially in contact with the middle one of the three transparencies10, and is consequently exposed with light values representing thetransmission values of the middle transparency 10 corrected by the lightvalues of the corresponding elements of the correction light mask 8 onthe face of the cathode ray tube. Diifusion plates 20 are placed behindthe outside transparencies 10, the difiusion produced by these platesbeing equal to that produced by the photographic plate 18, and thelatter is provided with a backing filter 22 which prevents exposure ofthe plate 18 to light of a colour to which it is sensitive Whilepermitting the passage of light of a colour to which the plate 18 isinsensitive. In producing the three colour printer plates eachseparation transparency in turn is placed in the centre position, andthe transparencies which are to be used to correct the printer plate areplaced in the outer positions.

In the example which is being described, to produce the magenta printerplate, the green filter separation negative G is placed in the centreposition in contact with the plate 18 to be exposed, as showndiagrammatically in Figure 2. On one side is placed the red filter (cyanprinter) separation negative R which alone is used to provide colourcorrection for the magenta printer. On the other side is placed theundercolour removal negative U, which is used to produce undercolourremoval for the magenta printer. By adjusting the amounts of cyan andundercolour removal signal which are added in the computer 14 to providethe modulating signal for the light source 6, satisfactory colourcorrection and undercolour removal can be produced for the magenta. Thecyan correction signal may be increased above the normal colourcorrection value (which may in a typical case be about 30%) to provide,some undercolour removal as well as colour correction. In terms-ofpercentage signal measured on the grey scale, there may be 4 40%subtraction of the cyan signal and 20% subtraction of the undercolourremoval signal.

In the case of the yellow printer, the blue filter separation negative Bis placed inthe centre position, the green filter (magenta printer)separation negative G is placed on one side for the purpose of "colourcorrection, .and the white light undercolour removal negative U isplaced on the other side to provide undercolour removal, as showndiagrammatically in Figure 3. Again, there may be 40% substraction ofthe magenta signal and 20% subtraction of the signal from the negativeU.

The cyan printer does not need to have a large colour correction signalfrom one of the other colours, and it is therefore used to correctitself. The red filter (cyan printer) negative R is placed in the centreposition (Figure 4) and a signal'is fed back from the centrephotomultiplier (cyan channel) to reduce itself to approximately thesame extent as the magenta and yellow were reduced by their correctingcolours. The white light negative U is again used for undercolourremoval. Suitable subtraction values are again 40% from the cyannegative R and 20% from the negative U.

In cases where the feedback required for the correction of the yellowand magenta plates is different, compensation for the plate requiringthe smaller correction can similarly be obtained by utilising the centrephotomultiplier to feed back a signal derived from this plate in such asense that the signal reduces itself. If the total correction ismaintained the same for each plate, then the separations produced canall be given the same photographic development treatment.

To produce the black printer plate, the white light negative U is usedin the centre position (Figure 5) and the red filter (cyan printer) andblue filter (yellow printer) negatives R and B are used in the two sidepositions. These two signals are fed back to the cathode ray tubetogether to correct the black printer, that is to say to remove densityfrom the black printer in the coloured areas of the picture. Suitablesubtraction values are 30% from each of the negatives R and B.

In one modification of this method, the cyan printer separation negativeR is used for undercolour removal instead of the white light negative Uin the production of the magenta and yellow printers. This means that inthe case ofthe magenta printer (Figure 2) a correction signal,sufiicient to give colour correction, is produced from the cyannegative, and then a further percentage of cyan channel signal is addedto the correction signal to give the required amount of undercolourremoval, the righthand position being unused. In the case of the yellowprinter, the magenta signal is still used for colour correction, but itssubtraction value may be increased to produce some undercolour removal,the remainder being effected by the cyan signal. The amount of colourremoved by the cyan signal is, of course, greater than the amount whichwould be removed if the cyan signal were used only for further colourcorrection of the magenta printer in a three-colour system in which noundercolour removal is carried out. The cyan printer and black printerplates are produced in the same was as before.

It will be clear that if desired the apparatus of Figure 1 can bemodified by the provision of a fourth separation negative position, andthat two of these four positions can then be used for colour-correctingseparation negatives while another is used for the white light negativefor undercolour removal. The remaining position is, of course, thatwhich receives the separation negative to be corrected in contact withthe plate to be exposed.

The method according to the invention can also be used with other knownforms of scanning and exposure apparatus, for example rotating-drum andoscillatingtable scanners, and apparatus in which separate analysing andreproducing scanners are used.

As well as colour correction and undercolour removal for each plate,modification to the tonal characteristic curve of the plate cansimultaneously be made by feeding signals through a separate channel asdescribed in copending application No. 826,764, filed July 13, 1959. Inthe case of the colour separation positive plates, when undercolourremoval is being applied, the required tonal modification is made to ascale of densities representing the total range of the colour (afterundercolour removal) to be printed from that positive. In the case ofthe black printer positive, the modification is adjusted so that when itis combined with the colour positives in the printing process, the scanof greys for the subject is also correct.

I claim:

1. In a colour reproduction system of the type using a black printer,the method of producing at least one of the colour printers whichincludes the following steps, placing an unexposed photographic emulsionfrom which the colour printer is to be derived behind a transparencyrepresenting the image, scanning the transparency by means of a lightsource in an electro-optical scanner to expose the photographic emulsionto light variations representing the corresponding colour componentimage, simultaneously scanning with light from the same light source atleast one transparency representing the image to obtain a scanner outputsignal representing colour correction to be applied to the colourprinter and a scanner output signal representing undercolour to beremoved from the colour printer, the undercolour removal signal beingalways derived from the same transparency, adding the said scanneroutput signals to produce a resultant correcting signal, and modulatingthe light source with the said resultant signal, whereby the saidphotographic emulsion is exposed to light values which include, inaddition to the modulation due to the transparency behind which thephotographic emulsion is placed, variations representing colourcorrection and undercolour removal.

2. A method according to claim 1, in which the transparency which isscanned to provide the undercolour removal signal is obtained byexposure of an original colour transparency to light which issubstantially white.

3. A method according to claim 1, in which the photographic emulsion tobe exposed is placed behind and substantially in contact with thetransparency through which it is to be exposed.

4. A method according to claim 1, in which the transparency which isscanned to provide the undercolour removal signal is the cyan printerseparation transparency.

5. In a colour reproduction system of the type using a black printer,the method of producing at least one of the colour printers whichincludes the following steps, placing an unexposed photographic emulsionfrom which the colour printer is to be derived behind a transparencyrepresenting the image, scanning the transparency by means of a lightsource in an electro-optical scanner to expose the photographic emulsionto light variations representing the corresponding colour componentimage, simultaneously scanning with light from the same light source atransparency representing the image to obtain a colour channel signal,amplifying the colour channel signal to an extent greater than thatrequired for colour correction and modulating the light source with theamplified signal, the photographic emulsion being thereby exposed tolight values which include, in addition to the modulation due to thetransparency behind which the photographic emulsion is placed,variations due to the light source modulation which represent colourcorrection and a degree of undercolour removal and which are alwaysderived from the scanning of the same transparency.

6. A method according to claim 5, in which the transparency which isscanned to provide a signal which, when amplified, serves both forcolour correction and for undercolour removal, is the cyan printerseparation transparency.

7. A method according to claim 5, in which the transparency which isscanned to provide a signal which, when amplified, serves both forcolour correction and for undercolour removal, is the magenta printerseparation transparency.

8. In a colour reproduction system of the type employing a blackprinter, the method of obtaining the black printer for the image to bereproduced including the following steps, placing an unexposedphotographic emulsion from which the black printer is to be derivedbehind a transparency representing the colour image, scanning thetransparency by means of a light source in an electrooptical scanner toexpose the photographic emulsion to light variations representing thetransmission values of the multi-coloured image, simultaneously scanningwith light from the same light source at least one transparencyrepresenting the image to obtain at least one scanner output signalrepresenting a colour component of the multicolour image, and modulatingthe light source with the said colour-component signals, whereby thesaid photo graphic emulsion is exposed to light variations whichinclude, in addition to the modulation due to the transparency behindwhich the photographic emulsion is placed, variations acting in a sensesuch as to reduce the density of a positive image resulting from theexposure.

Hall July 15, 1941 Rydz May 5, 1959

